Mass reduction in vehicle parts and components lowers the rate of the vehicle's fuel consumption. Thus different materials are considered to form vehicle structure which may be lighter in weight than steel or metal, but yet provide the sufficient structural strength for body support. In particular, it is known to replace metal frames for vehicle components such as seat assemblies with polyurethane, or polypropylene.
Polypropylene has sufficient compression and tensile strength to replace the metal in the manufacture of the seat back assembly frame. Indeed, some vehicle seat backs have done so. Expanded polypropylene (EPP) is generally polypropylene that has been expanded using gas injection during the molding process. EPP has a lower density than polypropylene and thus weighs less than the same volume of polypropylene. Accordingly, it remains desirable to reduce the mass of a seat back assembly by using a frame than is lighter than metal, but yet has sufficient structural integrity so as to withstand the load associated with vehicle operation.
Another design consideration for seat back assemblies is providing comfort to the user. For instance, it is known to provide seat back assemblies with adjustable headrests to accommodate occupants of different sizes. However such headrests require sufficient support along the base of the headrest so as to remain rigidly fixed to the seat back assembly. Thus, it is known to fixed adjustable headrests to a metal seat frame. The metal seat frame has a higher density relative to material such as polypropylene. Thus a vehicle with seat back assembly having a metal frame is heavier and consumes more fuel compared to that same vehicle with a seat back assembly having a frame made of polypropylene. Accordingly, it also remains desirable to have a relatively light weight seat back assembly with an adjustable head rest that is rigidly supported.